Inbred Lewis rat embryo cells will be transformed by Rous Sarcoma Virus and temperature sensitive mutants of Rous Sarcoma Virus. Revertants will be selected on the basis of selective techniques for growth or cell attachment. These revertants will be characterized for expression of viral genes using virus rescue procedures. Potential cell mutant function will be characterized and tested for complementation by a cell-hybridization test. Tumorigenicity of these cell lines in Lewis rats will be tested. A quantitative technique for rescue of Rous Sarcoma Virus from previously non-virogenic mammalian cells has been developed. This technique will be exploited to investigate the mechanism of rescue and identify the defect in the "non-virogenic" lines. Chicken embryo chondroblasts and pigmented retinal cells have been transformed by temperature-sensitive mutants of Rous Sarcoma Virus. Other specific types of chicken embryo cells will also be transformed. We will investigate the common properties of the various transformed cell types and examine the effect of cell-transformation on the expression of the specific differentiated cell programs.